(a) Field of the invention
This invention relates to a transmission type illuminating device for stereomicroscopes.
(b) Description of the prior art
In such conventional transmission type illuminating device of the kind as is disclosed, for example, in Japanese Utility Model Laid-Open No. 5808/1966, a frosted glass is arranged in a position conjugate with the entrance pupil of a stereomicroscope with respect to a condensor lens and is illuminated from the back to be used as a secondary light source so that a so-called Kohler illumination may be made. However, generally, in the Kohler illumination, unless the intensity of the beam emitted from the light source is constant irrespective of the direction, a sample will not be able to be uniformly illuminated. However, there has been a problem that, in such case of a light source consisting of a frosted glass illuminated from the back as in this conventional example, the beam intensity will vary so much depending on the direction that the illumination will fluctuate. There has been also a problem that, if the surface of the frosted glass is made rougher, the illumination fluctuation will tend to reduce but, in such case, the illumination efficiency will reduce and a bright illumination will be difficult. There has been also a problem that, as there are two entrance pupils in the stereomicroscope, in order to make a Kohler illumination which is an ideal, the illuminating optical system will become complicated. Further, there has been also a problem that, though most of the stereomicroscopes have magnification varying mechanisms, the position and aperture of the entrance pupil will vary with the magnification variation. Therefore, the Kohler illumination forming the image of the light source in the entrance pupil of the observation system will become imperfect. There has been also a problem that particularly, on the high magnification side, the F number will increase so much that the observed image will become dark but, in the above mentioned conventional example, in such case, no sufficient observation light amount will be able to be secured. There has been also a problem that, in a simple objective type stereomicroscope, the magnification can be varied by varying the focal distance by replacing the objective lens or by adding an adapter lens but, in such case, the position of the entrance pupil will move so much that the position conjugate with it will deviate from the frosted glass and therefore, in the above mentioned conventional example, only when an objective lens having a specific focal distance is used, the stereomicroscope will be able to be used.
Further, in some conventional transmission type illuminating devices, in order that even a transparent sample may be observed with a high contrast, the general bright visual field illumination can be switched over to an oblique light illumination. It is so formed that the illumination light is diffused by illuminating a frosted glass from the back with a light source A and collector lens B and the diffused light is reflected with a mirror D to illuminate a sample on a stage glass E and, when an oblique light illumination is to be made,the mirror D is rotated by a proper amount with the point O as a center to incline the illuminating light entering the stage glass E. However, there has been a problem that, in the case of this conventional example, the light diffusing force of the frosted glass C can not be made strong enough to obtain a large oblique light illumination effect, the effective aperture of the collector lens B can not be made large due to the restriction in fitting and thereby particularly, in the case of an oblique light illumination, only an illumination in a limited range can be made. In the transmission type illuminating device desclosed in the above mentioned Japanese Utility Model Laid-Open No. 5808/1966, an oblique light illumination is realized by knife-edging a part of the above mentioned frosted glass to interrupt the light so as to enter only a part of the entrance pupil. However, in order to obtain a uniform illumination effect with the Kohler illumination, the intensity of the light emitted from the secondary light source must be constant irrespective of the direction and therefore, in such case that the frosted glass is a secondary light source as in the conventional example, it will be necessary to make the diffusing force of the frosted glass considerably strong. Therefore, there has been a problem that the illumination efficiency will be so low that no bright illumination will be obtained and further, in the oblique light illumination, as the light is interrupted by knife-edging a part of the frosted glass which is a secondary light source, the light amount will reduce so much that no sufficient brightness will be obtained.